User terminal and base station

ABSTRACT

A user terminal exists in a first cell operating at a first frequency, in a mobile communication system that supports a D2D (Device to Device) proximity service. The user terminal includes a transmitter configured to transmit a D2D interest indication indicating that the user terminal has an interest in the D2D proximity service, to a base station forming the first cell.

RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/048,056, filed Feb. 19, 2016, which is acontinuation application of international application PCT/JP2015/063261,filed May 8, 2015, which claims benefit of U.S. provisional application61/990,936, filed May 9, 2014, the entirety of both applications herebyexpressly incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a user terminal and a base stationthat are used in a mobile communication system.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project) which is a project aimingto standardize a mobile communication system, the introduction of aDevice-to-Device (D2D) proximity service is discussed as a new functionon and after Release 12 (see Non Patent Document 1).

The D2D proximity service (D2D ProSe) is a service in which directdevice-to-device communication is provided within a synchronizationcluster formed by a plurality of user terminals which are synchronizedto one another. The D2D proximity service includes a discovery procedure(Discovery) in which a proximal terminal is discovered and D2Dcommunication (Communication) that is direct Device-to-Devicecommunication.

A discovery procedure, in which a user terminal that exists in a firstcell discovers a proximal terminal that exists in a second cell which isprovided around the first cell, is called an inter-cell discoveryprocedure (Inter-cell discovery). D2D communication in which a userterminal that exists in the first cell performs communication with aproximal terminal that exists in the second cell is called inter-cellD2D communication (Inter-cell communication).

However, in a case where a frequency employed in the first cell and afrequency employed in the second cell differ, even when the userterminal transmits a discovery signal in order to discover the proximalterminal in the Inter-cell discovery, the proximal terminal is notcapable of receiving the discovery signal. Therefore, in such a case, aprocedure in which the Inter-cell discovery is appropriately performedis desired.

PRIOR ART DOCUMENTS Non Patent Document

-   [Non Patent Document 1] 3GPP technical report “TR 36.843 V12.0.1”    March, 2014

SUMMARY

A first aspect is summarized as a user terminal that exists in a firstcell operating at a first frequency in a mobile communication systemthat supports a D2D proximity service, the user terminal including: atransmitter configured to transmit a discovery signal for discovering aproximal terminal that exists in a second cell different from the firstcell by using radio resources of a second frequency different from thefirst frequency, wherein the radio resources of the second frequency areradio resources used exclusively for an uplink.

A second aspect is summarized as a base station forming at least a firstcell operating at a first frequency, including: a controller configuredto assign, to a user terminal that exists in the first cell, radioresources of a second frequency different from the first frequency,wherein the radio resources of the second frequency are radio resourcesused exclusively for an uplink.

A third aspect is summarized as a user terminal that exists in a firstcell operating at a first frequency in a mobile communication systemthat supports a D2D proximity service, the user terminal including: atransmitter configured to transmit a D2D interest indication forindicating an interest in the D2D proximity service, to a base stationforming the first cell.

A fourth aspect is summarized as a user terminal that exists in a firstcell operating at a first frequency, in a mobile communication systemthat supports a D2D proximity service, the user terminal including: afirst receiver configured to perform reception at the first frequencywhich is a serving frequency; a second receiver configured to performreception at a second frequency different from the first frequency; anda controller configured to monitor a discovery signal by using thesecond receiver, the discovery signal transmitted by another userterminal at the second frequency.

A fifth aspect is summarized as a user terminal that exists in a firstcell operating at a first frequency, in a mobile communication systemthat supports a D2D proximity service, the user terminal including: afirst transmitter configured to perform transmission at the firstfrequency which is a serving frequency; a second transmitter configuredto perform transmission at a second frequency different from the firstfrequency; and a controller configured to perform a process oftransmitting a discovery signal at the second frequency by using thesecond transmitter, to other user terminal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram of an LTE system according to a firstembodiment.

FIG. 2 is a block diagram of a UE 100 according to the first embodiment.

FIG. 3 is a block diagram of an eNB 200 according to the firstembodiment.

FIG. 4 is a protocol stack diagram of a radio interface according to thefirst embodiment.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem according to the first embodiment.

FIG. 6 is a diagram illustrating an operation environment according tothe first embodiment.

FIG. 7 is a diagram illustrating an operation environment according tothe first embodiment.

FIG. 8 is a diagram illustrating an operation environment according tothe first embodiment.

FIG. 9 is a sequence diagram illustrating an operation according to thefirst embodiment.

FIG. 10 is a sequence diagram illustrating an operation according to thefirst embodiment.

FIG. 11 is a sequence diagram illustrating an operation according to afirst modification.

FIG. 12 is a sequence diagram illustrating an operation according to athird modification.

FIG. 13 is a sequence diagram illustrating an operation according to thethird modification.

FIG. 14 is a diagram illustrating an operation environment according toa fourth modification.

DESCRIPTION OF EMBODIMENTS Overview of Embodiments

A user terminal according to an embodiment exists in a first celloperating at a first frequency, in a mobile communication system thatsupports a D2D proximity service. The user terminal includes atransmitter configured to transmit a discovery signal for discovering aproximal terminal that exists in a second cell different from the firstcell by using radio resources of a second frequency different from thefirst frequency. The radio resources of the second frequency are radioresources used exclusively for an uplink.

In the embodiment, a new concept of “radio resources of the secondfrequency used exclusively for an uplink” are introduced, and the userterminal transmits the discovery signal by using the radio resources ofthe second frequency. As a result, even in a case where the frequencyemployed in the first cell and the frequency employed in the second celldiffer, the proximal terminal is capable of receiving the discoverysignal to enable appropriate execution of the Inter-cell discovery.

A user terminal according to the embodiment exists in a first celloperating at a first frequency, in a mobile communication system thatsupports a D2D proximity service. The user terminal includes atransmitter configured to transmit a D2D interest indication indicatingthat the user terminal has an interest in the D2D proximity service, toa base station forming the first cell.

In the embodiment, a new concept of “D2D interest indication indicatingthat the user terminal has an interest in the D2D proximity service” isintroduced, and the user terminal transmits the D2D interest indicationto the base station forming the first cell. In other words, the basestation is provided with decision-making information for performing atransition (handover or cell reselection) of the user terminal towardthe frequency at which the D2D proximity service is provided. Therefore,even when the frequencies of the cells in which a plurality of userterminals exist differ from one another, it is possible to match thefrequencies of the cells in which the plurality of user terminals existto a frequency at which the D2D proximity service is provided, and it ispossible to appropriately perform Inter-cell discovery.

First Embodiment

A first embodiment will be described by using, as an example, an LTEsystem based on 3GPP standards as a mobile communication system, below.

(1) System Configuration

First of all, the system configuration of LTE system according to afirst embodiment will be described. FIG. 1 is a configuration diagram ofthe LTE system according to the first embodiment.

As illustrated in FIG. 1, the LTE system according to the firstembodiment includes UE (User Equipment) 100, E-UTRAN (Evolved-UMTSTerrestrial Radio Access Network) 10, and EPC (Evolved Packet Core) 20.

The UE 100 corresponds to a user terminal. The UE 100 is a mobilecommunication device, which performs radio communication with a cell (aserving cell in a case where the UE 100 is in a connected state) formedby the eNB 200. The configuration of the UE 100 will be described later.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes eNB 200 (an evolved Node-B). The eNB 200 corresponds to a basestation. The eNBs 200 are connected mutually via an X2 interface. Theconfiguration of the eNB 200 will be described later.

The eNB 200 manages one or a plurality of cells, and performs radiocommunication with the UE 100 that establishes a connection with a cellof the eNB 200. The eNB 200 has a radio resources management (RRM)function, a routing function of user data, a measurement controlfunction for mobility control and scheduling and the like. The “cell” isused as a term indicating a smallest unit of a radio communication area,and is also used as a term indicating a function of performing radiocommunication with the UE 100.

The EPC 20 corresponds to a core network. The EPC 20 includes MME(Mobility Management Entity)/S-GW (Serving-Gateway) 300. The MMEperforms different types of mobility control and the like for the UE100. The S-GW performs transfer control of the user data. The MME/S-GW300 is connected to the eNB 200 via an S1 interface. It is noted thatthe E-UTRAN 10 and the EPC 20 constitute a network of the LTE system.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, theUE 100 includes a plurality of antennas 101, a radio transceiver 110, auser interface 120, a GNSS (Global Navigation Satellite System) receiver130, a battery 140, a memory 150, and a processor 160. The memory 150and the processor 160 constitute a controller. The radio transceiver 110and the processor 160 constitute a transmitter and a receiver. The UE100 may not necessarily have the GNSS receiver 130. Furthermore, thememory 150 may be integrally formed with the processor 160, and this set(that is, a chip set) may be called a processor 160′.

The antenna 101 and the radio transceiver 110 are used to transmit andreceive a radio signal. The radio transceiver 110 converts a basebandsignal (a transmission signal) output from the processor 160 into aradio signal, and transmits the radio signal from the antenna 101.Furthermore, the radio transceiver 110 converts a radio signal receivedby the antenna 101 into a baseband signal (a reception signal), andoutputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user carrying the UE 100,and includes, for example, a display, a microphone, a speaker, andvarious buttons. The user interface 120 receives an operation from auser and outputs a signal indicating the content of the operation to theprocessor 160. The GNSS receiver 130 receives a GNSS signal in order toobtain location information indicating a geographical location of the UE100, and outputs the received signal to the processor 160. The battery140 accumulates a power to be supplied to each block of the UE 100.

The memory 150 stores a program to be executed by the processor 160 andinformation to be used for processing by the processor 160. Theprocessor 160 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signal,and a CPU (Central Processing Unit) that performs various types ofprocesses by executing the program stored in the memory 150. Theprocessor 160 may further include a codec that performs encoding anddecoding on sound and video signals. The processor 160 executes varioustypes of processes and various types of communication protocolsdescribed later.

FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, theeNB 200 includes a plurality of antennas 201, a radio transceiver 210, anetwork interface 220, a memory 230, and a processor 240. The memory 230and the processor 240 constitute a controller. The radio transceiver 210and the processor 240 constitute a transmitter and a receiver.Furthermore, the memory 230 may be integrally formed with the processor240, and this set (that is, a chipset) may be called a processor.

The antenna 201 and the radio transceiver 210 are used to transmit andreceive a radio signal. The radio transceiver 210 converts a basebandsignal (a transmission signal) output from the processor 240 into aradio signal, and transmits the radio signal from the antenna 201.Furthermore, the radio transceiver 210 converts a radio signal receivedby the antenna 201 into a baseband signal (a reception signal), andoutputs the baseband signal to the processor 240.

The network interface 220 is connected to the neighboring eNB 200 viathe X2 interface, and is connected to the MME/S-GW 300 via the S1interface. The network interface 220 is used in communication performedon the X2 interface and communication performed on the S1 interface.

The memory 230 stores a program to be executed by the processor 240 andinformation to be used for processing by the processor 240. Theprocessor 240 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signaland a CPU that performs various types of processes by executing theprogram stored in the memory 230. The processor 240 executes varioustypes of processes and various types of communication protocolsdescribed later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isclassified into a first layer to a third layer of an OSI referencemodel, such that the first layer is a physical (PHY) layer. The secondlayer includes a MAC (Media Access Control) layer, an RLC (Radio LinkControl) layer, and a PDCP (Packet Data Convergence Protocol) layer. Thethird layer includes an RRC (Radio Resource Control) layer.

The physical layer performs encoding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Between the physical layer of the UE 100 and the physicallayer of the eNB 200, user data and control signals are transmitted viaa physical channel.

The MAC layer performs priority control of data, a retransmissionprocess by a hybrid ARQ (HARQ), a random access procedure, and the like.Between the MAC layer of the UE 100 and the MAC layer of the eNB 200,user data and control signals are transmitted via a transport channel.The MAC layer of the eNB 200 includes a scheduler for determining atransport format (a transport block size and a modulation and codingscheme) of an uplink and a downlink, and resource blocks to be assignedto the UE 100.

The RLC layer transmits data to an RLC layer of a reception side byusing the functions of the MAC layer and the physical layer. Between theRLC layer of the UE 100 and the RLC layer of the eNB 200, user data andcontrol signals are transmitted via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption.

The RRC layer is defined only in a control plane that handles controlsignals. Between the RRC layer of the UE 100 and the RRC layer of theeNB 200, a control signal (an RRC message) for various types of settingsis transmitted. The RRC layer controls a logical channel, a transportchannel, and a physical channel according to the establishment,re-establishment, and release of a radio bearer. When there is aconnection (an RRC connection) between the RRC of the UE 100 and the RRCof the eNB 200, the UE 100 is in an RRC connected state. Otherwise, theUE 100 is in an RRC idle state.

An NAS (Non-Access Stratum) layer positioned above the RRC layerperforms session management, mobility management and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, OFDMA (Orthogonal Frequency DivisionMultiplexing Access) is applied to a downlink, and SC-FDMA (SingleCarrier Frequency Division Multiple Access) is applied to an uplink,respectively.

As illustrated in FIG. 5, a radio frame is configured by 10 subframesarranged in a time direction. Each subframe is configured by two slotsarranged in the time direction. Each subframe has a length of 1 ms andeach slot has a length of 0.5 ms. Each subframe includes a plurality ofresource blocks (RBs) in a frequency direction (not shown), and aplurality of symbols in the time direction. Each resource block includesa plurality of subcarriers in the frequency direction. One symbol andone subcarrier forms one resource element. Of the radio resources (timeand frequency resources) assigned to the UE 100, a frequency resourcecan be identified by a resource block and a time resource can beidentified by a subframe (or a slot).

(2) D2D Proximity Service

A D2D proximity service will be described, below. An LTE systemaccording to the first embodiment supports the D2D proximity service.

The D2D proximity service (D2D ProSe) is a service enabling directUE-to-UE communication within a synchronization cluster formed by aplurality of UEs 100 which are synchronized to one another. The D2Dproximity service includes a discovery procedure (Discovery) in which aproximal UE is discovered and D2D communication (Communication) that isdirect UE-to-UE communication. The D2D communication is also calledDirect communication.

A scenario in which all the UEs 100 forming the synchronization clusterare located inside the coverage of at least one cell is called “Incoverage”. A scenario in which all the UEs 100 forming thesynchronization cluster are located outside a coverage of at least onecell is called “Out of coverage”. A scenario in which some UEs 100, outof the plurality of UEs 100 forming the synchronization cluster, arelocated in a coverage of at least one cell and the remaining UEs 100 arelocated outside a coverage of at least one cell is called “Partialcoverage”.

In an “In coverage” scenario, the eNB 200 is a D2D synchronizationsource. A D2D non-synchronization source, from which a D2Dsynchronization signal is not transmitted, is synchronized with the D2Dsynchronization source. The eNB 200 that is a D2D synchronization sourcetransmits a broadcast signal including D2D resource informationindicating radio resources (resource pool) available for the D2Dproximity service. The D2D resource information includes informationindicating a resource pool for the discovery procedure (Discoveryresource information) and information indicating a resource pool for theD2D communication (Communication resource information), for example. TheUE 100 that is a D2D non-synchronization source performs the discoveryprocedure and the D2D communication on the basis of the D2D resourceinformation received from the eNB 200.

In “Out of coverage” or “Partial coverage”, the UE 100 is a D2Dsynchronization source. In “Out of coverage”, the UE 100 that is a D2Dsynchronization source transmits D2D resource information indicatingradio resources (resource pool) available for the D2D proximity service.The D2D resource information is included in the D2D synchronizationsignal, for example. The D2D synchronization signal is a signaltransmitted in the synchronization procedure in which a device-to-devicesynchronization is established. The D2D synchronization signal includesa D2D SS and a physical D2D synchronization channel (PD2DSCH). The D2DSS is a signal for providing a synchronization standard of a time and afrequency. The PD2DSCH is a physical channel through which a greateramount of information can be carried than can be carried through the D2DSS. The PD2DSCH carries the above-described D2D resource information(the Discovery resource information and the Communication resourceinformation). Alternatively, when the D2D SS is previously associatedwith the D2D resource information, the transmission of the PD2DSCH maybe omitted.

The discovery procedure is used mainly when the D2D communication isperformed by unicast. In a case where a first UE 100 starts D2Dcommunication with a second UE 100, the first UE 100 uses any particularradio resource out of radio resources for the discovery procedure totransmit the discovery signal. On the other hand, in a case where thesecond UE 100 starts the D2D communication with the first UE 100, thesecond UE 100 scans the discovery signal within the resource pool forthe discovery procedure to receive the discovery signal. The discoverysignal may include information indicating radio resources used by thefirst UE 100 for the D2D communication.

Further, a discovery procedure in which a user terminal that exists inthe first cell discovers the proximal terminal that exists in a secondcell which is provided around the first cell is called an inter-celldiscovery procedure (Inter-cell discovery). D2D communication in which auser terminal that exists in the first cell performs communication withthe proximal terminal that exists in the second cell is calledinter-cell D2D communication (Inter-cell communication).

(3) Operation Environment

An operation environment according to the first embodiment will bedescribed, below. FIG. 6 to FIG. 8 are diagrams illustrating anoperation environment according to the first embodiment. As illustratedin FIG. 6 to FIG. 8, there are three options for an operationenvironment according to the first embodiment.

The three operation environments illustrated in FIG. 6 to FIG. 8 havethe following in common:

A UE 100#1 exists in a cell #1. The UE 100#1 is in an RRC connectedstate or an RRC idle state in the cell #1. For the UE 100#1, the cell #1is a serving cell and a cell #2 is a neighboring cell.

A UE 100#2 exists in the cell #2. The UE 100#2 is in an RRC connectedstate or an RRC idle state in the cell #2. For the UE 100#2, the cell #1is a neighboring cell and the cell #2 is a serving cell.

In a first operation environment, as illustrated in FIG. 6, an eNB 200#1forms the cell #1 operating at a frequency f1 and the cell #2 operatingat a frequency f2.

In a second operation environment, as illustrated in FIG. 7, the eNB200#1 forms the cell #1 operating at the frequency f1, and an eNB 200#2forms the cell #2 operating at the frequency f2. The eNB 200#2 is a picocell or a femto cell, and a coverage of the cell #2 overlaps a coverageof the cell #1. A whole of the coverage of the cell #2 may overlap thecoverage of the cell #1, or a part of the coverage of the cell #2 mayoverlap the coverage of the cell #1.

In a third operation environment, as illustrated in FIG. 8, the eNB200#1 forms the cell #1 operating at the frequency f1, and the eNB 200#2forms the cell #2 operating at the frequency f2. The eNB 200#2 is amacro cell and the cell #2 is arranged by partially overlapping with thecell #1.

In the first embodiment, in such an operation environment, a scenario isassumed where the Inter-cell discovery is performed in which the UE100#1 discovers the UE 100#2. In such an operation environment, evenwhen the UE 100#1 transmits the discovery signal at the frequency f1 inthe Inter-cell discovery, the frequency of the cell #2 in which the UE100#2 exists is the frequency f2, and thus, the UE 100#2 is not capableof receiving the discovery signal.

Therefore, in the first embodiment, the UE 100#1 uses the radioresources of the frequency f2 different from the frequency f1 totransmit the discovery signal for discovering the UE 100#2 that existsin the cell #2 different from the cell #1. Here, it should be noted thatthe radio resources of the frequency f2 are radio resources usedexclusively for an uplink.

Here, it is preferable that the UE 100#1 uses synchronizationinformation used in the cell #1 to transmit the discovery signal. Inother words, it is preferable that the UE 100#1 transmits the discoverysignal at a timing synchronized with the eNB 200#1 forming the cell #1.

Further, the UE 100#1 may transmit, to the eNB 200#1 forming the cell#1, an assignment request for requesting an assignment of the radioresources of the frequency f2. The eNB 200#1 assigns radio resources ofthe frequency f2 to the UE 100#1, as radio resources (resource pool)available for the D2D proximity service, in response to a signalreceived from the UE 100#1.

In such a scenario, there may be two options as an option in which theUE 100#1 uses the radio resources of the frequency f2.

In a first option, the radio resources of the frequency f2 areconfigured as radio resources of a secondary cell of the UE100#1 whenthe cell #1 is configured as a primary cell of the UE 100#1. Thesecondary cell is operating at the frequency f2, is dedicated to theuplink, and is configured by the eNB 200#1 forming the cell #1. Itshould be noted that in the first option, the eNB 200#1 forming the cell#1 does not need to provide the uplink and the downlink at the frequencyf2. That is, the secondary cell may be a cell used exclusively for theInter-cell discovery. However, the first option is not limited thereto,and the eNB 200#1 may form a cell operating at the frequency f2.

In a second option, the radio resources of the frequency f2 is scheduledby downlink control information (DCI: Downlink Control Information)dedicated to the D2D proximity service. The downlink control informationis transmitted from the eNB 200#1 forming the cell #1. It is noted that,the downlink control information is transmitted by using the frequencyf1. It should be noted that in the second option, the eNB 200#1 formingthe cell #1 does not need to provide the uplink and the downlink at thefrequency f2. That is, the secondary cell may be a cell used exclusivelyfor the Inter-cell discovery. However, the second option is not limitedthereto, and the eNB 200#1 may form a cell operating at the frequencyf2.

(4) Operation According to First Embodiment

An operation according to the first embodiment will be described, below.The above-described first option and second option will be described,below.

(4.1) First Option

FIG. 9 is a sequence diagram illustrating the first option according tothe first embodiment. It should be noted that in FIG. 9, the operationenvironment illustrated in FIG. 6 is a prerequisite.

As illustrated in FIG. 9, in step S11, the UE 100#1 transmits, to theeNB 200#1 forming the cell #1, an assignment request for requesting anassignment of the radio resources of the frequency f2.

In step S12, the eNB 200#1 transmits, to the UE 100#1, a message(Configuration message) for configuring a secondary cell of the UE100#1, when the cell #1 is configured as a primary cell of the UE 100#1.As described above, it should be noted that the eNB 200#1 forming thecell #1 does not need to provide the uplink and the downlink at thefrequency f2.

That is, the eNB 200#1 assigns, as radio resources used exclusively forthe D2D proximity service (radio resource used exclusively for theuplink), radio resources of the frequency f2, to the eNB 200#1. In thefirst embodiment, the radio resources of the frequency f2 are used fortransmitting a discovery signal for discovering the UE 100#2 that existsin the cell #2 (frequency f2) different from the cell #1 (frequency f1),in the D2D proximity service.

In step S13, the UE 100#1 uses radio resources of the frequency f2different from the frequency f1 to transmit a discovery signal fordiscovering the UE 100#2 that exists in the cell #2 different from thecell #1. As described above, it is preferable that the UE 100#1 uses thesynchronization information used in the cell #1 to transmit thediscovery signal.

It is noted that the process in step S11 is not requisite, and the eNB200#1 may transmit the Configuration message to the UE 100#1 on thebasis of another trigger.

(4.2) Second Option

FIG. 10 is a sequence diagram illustrating the second option accordingto the first embodiment. It should be noted that in FIG. 10, theoperation environment illustrated in FIG. 6 is a prerequisite.

As illustrated in FIG. 10, in step S21, the UE 100#1 transmits, to theeNB 200#1 forming the cell #1, an assignment request for requesting anassignment of radio resources of the frequency f2.

In step S22, the eNB 200#1 transmits, to the UE 100#1, the downlinkcontrol information (DCI: Downlink Control Information) dedicated to theD2D proximity service. The downlink control information is informationfor scheduling the radio resources of the frequency f2. As describedabove, it should be noted that the eNB 200#1 forming the cell #1 doesnot need to provide the uplink and the downlink at the frequency f2.

That is, the eNB 200#1 assigns, as radio resources used exclusively forthe D2D proximity service (radio resource used exclusively for theuplink), radio resources of the frequency f2, to the eNB 200#1. In thefirst embodiment, the radio resources of the frequency f2 are used fortransmitting a discovery signal for discovering the UE 100#2 that existsin the cell #2 (frequency f2) different from the cell #1 (frequency f1)in the D2D proximity service.

In step S23, the UE 100#1 uses radio resources of the frequency f2different from the frequency f1 to transmit a discovery signal fordiscovering the UE 100#2 that exists in the cell #2 different from thecell #1. As described above, it is preferable that the UE 100#1 uses thesynchronization information used in the cell #1 to transmit thediscovery signal.

It is noted that the process in step S21 is not requisite, and the eNB200#1 may transmit, to the UE 100#1, the configuration message on thebasis of another trigger.

(5) Operation and Effect

In the first embodiment, a new concept of “radio resource of the secondfrequency used exclusively for an uplink” is introduced, and the UE100#1 transmits a discovery signal by using radio resources of thefrequency f2. As a result, even in a case where the frequency employedin the first cell and the frequency employed in the second cell differ,the UE 100#2 is capable of receiving the discovery signal to enableappropriate execution of the Inter-cell discovery.

[First Modification]

A first modification of the first embodiment will be described below.The description proceeds with a particular focus on a difference fromthe first embodiment.

Specifically, in the first embodiment, the UE 100#1 uses radio resourcesof the frequency f2 to transmit a discovery signal. On the other hand,in the first modification, through the transmission of a D2D interestindication indicating that the UE has an interest in the D2D proximityservice, the frequencies of cells in which a plurality of UEs 100 existare matched with the frequency at which the D2D proximity service isprovided.

Here, in the first modification, a case is provided as an example wherethe D2D proximity service is not provided at the frequency f2, and theD2D proximity service is provided at the frequency f1. In such a case, aprocedure is described in which the cell #2 in which the UE 100#2 existsis changed to a cell operating at the frequency f1. Thus, it should benoted that the first modification is a scenario in which the D2Dproximity service does not need to be provided at all the frequenciesprovided in the mobile communication system (WAN; Wide Area Network). Inother words, it should be noted that the UE 100 does not need to supportthe D2D proximity service at all the frequencies provided in the mobilecommunication system (WAN; Wide Area Network). The frequency at whichthe D2D proximity service is supported may differ depending on each UE100.

It is noted that in the first operation environment (see FIG. 6), thecell operating at the frequency f1 is a cell provided by the eNB 200#1.In the second operation environment (see FIG. 7), the cell operating atthe frequency f1 may be a cell provided by the eNB 200#1 and may be acell provided by the eNB 200#1. In a third operation environment (seeFIG. 8), the cell operating at the frequency f1 is a cell provided bythe eNB 200#2.

More particularly, as illustrated in FIG. 11, in step S31, the eNB 200#2notifies (broadcasts) information indicating the frequency at which theD2D proximity service is provided. Such information is notified by anSIB (System Information Block), for example.

In step S32, the UE 100#2 transmits, to the eNB 200#2, the D2D interestindication (D2D Interest Indication) that indicates an interest in theD2D proximity service. The UE 100#2 may refer to the informationreceived in step S31 and transmit the D2D interest indication to the eNB200#2 when the D2D proximity service is not provided at the frequencyf2. In other words, the UE 100#2 may transmit the D2D interestindication to the eNB 200#2 when existing on the cell (frequency) inwhich the D2D proximity service is not provided. However, the firstmodification is not limited thereto, and the UE 100#2 may transmit theD2D interest indication to the eNB 200#2 without referring to theinformation received in step S31.

In step S33, the eNB 200#2 transmits, to the UE 100#2, a message(transition instruction) for instructing a transition from the frequencyf2 (the cell #2) to the frequency f1 (a cell operating at the frequencyf1). When the UE 100#2 is in an RRC connected state, the transitioninstruction is a handover instruction from the cell #2 to a celloperating at the frequency f1. On the other hand, when the UE 100#2 isin an RRC connected state and the UE 100#2 is transitioned to an RRCidle state after the D2D interest indication is transmitted, thetransition instruction may be a cell change instruction (Cell ChangeOrder) from the cell #2 to the cell operating at the frequency f1. Insuch a case, the eNB 200#2 transmits, to the UE 100#2, for example, thecell change instruction (Cell Change Order) as well as an RRC connectionrelease message (RRC Connection Release).

It should be noted that the D2D interest indication is transmitted in anRRC connected state, and thus, in step S33, it is highly probable thatthe UE 100#2 is in an RRC connected state.

Further, it should be noted that in step S33, the UE 100#2 performstransition (handover or cell reselection) to the frequency f1 in whichthe D2D proximity service is provided.

In step S34, the UE 100#2 scans the discovery signal at the frequencyf1. As a result, the UE 100#2 is capable of receiving the discoverysignal transmitted from the UE 100#1.

Here, the D2D interest indication may include information indicating atype of D2D proximity services in which the UE 100#1 is interested. Atype of D2D proximity services may include information indicatingwhether or not the Inter-cell communication is desired and informationindicating a type of UEs 100 in which the Inter-cell communicationshould be performed.

(Operation and Effect)

In the first modification, a new concept of a “D2D interest indicationindicating an interest in the D2D proximity service” is introduced, andthe UE 100#2 transmits the D2D interest indication to the eNB 200#2forming the cell #2. In other words, the eNB 200 is provided withdecision-making information for performing a transition (handover orcell reselection) of the UE 100 relative to the frequency at which theD2D proximity service is provided. Therefore, even when the frequenciesof the cells on which a plurality of UEs 100 exist differ from oneanother, it is possible to match the frequencies of the cells on whichthe plurality of UEs 100 exist to a frequency (here, the frequency f1)in which the D2D proximity service is provided, and it is possible toappropriately perform the Inter-cell discovery.

[Second Modification]

A second modification of the first embodiment will be described, below.The description proceeds with a particular focus on a difference fromthe first modification.

Specifically, in the first modification, a case is described where theUE 100#2 transmits the D2D interest indication when the D2D proximityservice is not provided at the frequency f2 and the D2D proximityservice is provided at the frequency f1.

On the other hand, in the second modification, a case is provided, as anexample, where the D2D proximity service is not provided at thefrequency f1 and the D2D proximity service is provided at the frequencyf2. Further, it should be noted that the eNB 200#1 forms a celloperating at the frequency f2 in addition to the cell #1 operating atthe frequency f1.

Here, it should be noted that the second modification is a scenario inwhich the D2D proximity service does not need to be provided at all thefrequencies provided in the mobile communication system (WAN; Wide AreaNetwork), similarly to the first modification. In other words, it shouldbe noted that the UE 100 does not need to support the D2D proximityservice at all the frequencies provided in the mobile communicationsystem (WAN; Wide Area Network). The frequency at which the D2Dproximity service is supported may differ depending on each UE 100.

In such a case, in the second modification, the UE 100#1 transmits theD2D interest indication to the eNB 200#1 forming the cell #1. The UE100#1 may transmit the D2D interest indication to the eNB 200#1 whenexisting on the cell (frequency) in which the D2D proximity service isnot provided.

Here, the eNB 200#1 that receives the D2D interest indication preferablytransmits, to the UE 100#1, a message (transition instruction) forinstructing a transition from the frequency f1 to the frequency f2, asdescribed in a third modification described later. The UE 100#1 thatreceives the transition instruction preferably performs transition(handover or cell reselection) to the frequency f2 in which the D2Dproximity service is provided, as described in the third modificationdescribed later.

As a result, similarly to the first modification, even when thefrequencies of the cells on which a plurality of UEs 100 exist differfrom one another, it is possible to match the frequency of the cell onwhich the plurality of UEs 100 exist to a frequency (here, the frequencyf2) in which the D2D proximity service is provided, and it is possibleto appropriately perform the Inter-cell discovery.

It is noted that in the second modification, the information indicatingthe frequency at which the D2D proximity service is provided preferablyis notified from the eNB 200#1. Such information is notified by an SIB(System Information Block), for example.

[Third Modification]

A third modification of the first embodiment will be described, below.The description proceeds with a particular focus on a difference fromthe first embodiment and the second modification.

Specifically, in the first embodiment, an assignment process of theradio resources of the frequency f2 (step S12 or step S22) is triggeredby transmission of an assignment request for requesting assignment ofthe radio resources of the frequency f2 from the UE 100#1 to the eNB200#1.

On the other hand, in the third modification, the assignment process ofthe radio resources of the frequency f2 (step S12 or step S22) istriggered by transmission of the D2D interest indication from the UE100#1 to the eNB 200#1.

That is, in the above-described first option, as illustrated in FIG. 12,a process in step S11A is performed instead of step S11 illustrated inFIG. 9. In step S11A, the UE 100#1 transmits, to the eNB 200#1, the D2Dinterest indication indicating an interest in the D2D proximity service.Likewise, in the above-described second option, as illustrated in FIG.13, a process in step S21A is performed instead of step S21 illustratedin FIG. 10. In step S21A, the UE 100#1 transmits, to the eNB 200#1, theD2D interest indication indicating an interest in the D2D proximityservice.

[Fourth Modification]

A fourth modification of the first embodiment will be described, below.The description proceeds with a particular focus on a difference fromthe first embodiment.

Specifically, in the first embodiment, the radio resources usedexclusively for the uplink are the radio resources used exclusively forthe D2D proximity service.

On the other hand, in the fourth modification, radio resources usedexclusively for the uplink are used for transmitting a signal used forinforming the existence of the UE 100, to the eNB 200 that manages acell (for example, a pico cell or a femto cell) having a coveragenarrower than a coverage of a cell (for example, a macro cell) on whichthe UE 100 exists.

More particularly, as illustrated in FIG. 14, an eNB 200#11 forms a cell#11 (for example, a macro cell) operating at a frequency f11. An eNB200#12 forms a cell #12 (for example, a pico cell or a femto cell)operating at a frequency f12. The cell #12 has a coverage narrower thana coverage of the cell #11. The coverage of the cell #12 overlaps thecoverage of the cell #11. The UE 100#1 exists in the cell #11.

In such an operation environment, the eNB 200#11 assigns the radioresources of the second frequency f12 different from the frequency f11,to the UE 100#1 that exists in the cell #11. The radio resource of thesecond frequency f12 are used for transmitting a signal used forinforming the existence of the UE 100#1, to the eNB 200#12 forming thecell #12.

Here, as an example of such a signal, an RACH signal may be used. Whenthe UE 100#1 uses the radio resources of the second frequency f12 totransmit the RACH signal, it becomes possible for the UE 100#1 to informthe eNB 200#12 forming the cell #12 of the existence of the UE 100#1.

Other Embodiments

The present disclosure is explained through the above-describedembodiments, but it must not be understood that this disclosure islimited by the statements and the drawings constituting a part of thisdisclosure. From this disclosure, various alternative embodiments,examples, and operational technologies will become apparent to thoseskilled in the art.

For example in other embodiments, the UE 100 preferably transmits theD2D interest indication when the D2D function is turned on. The D2Dfunction is a function of receiving the D2D proximity service, andon/off of the D2D function is switched by a user operation, for example.On/off of the D2D function may be switched depending on whether or notthe UE 100 exists in a cell provided by a mobile communication system(WAN; Wide Area Network) rather than through the user operation.

For example in other embodiments, the D2D proximity service may beprovided at a plurality of frequencies. In such a case, the eNB 200 mayselect the frequency at which the D2D proximity service should beprovided out of a plurality of frequencies, in response to reception ofthe D2D interest indication. For example, the eNB 200 may select afrequency at which the D2D proximity service should be provided, inaccordance with a congestion situation of each frequency and aninterference situation of each frequency. Alternatively, the eNB 200 mayselect a frequency at which the D2D proximity service should be providedin accordance with a combination of UEs 100 that receive the D2Dproximity service when the D2D proximity service is performed inunicast. As a result, it is possible to perform scheduling in accordancewith a combination of UEs 100 that receive the D2D proximity service.

For example in other embodiments, the D2D interest indication mayinclude information indicating a frequency used for transmitting adiscovery signal, information indicating a frequency used for receivinga discovery signal, or information indicating a frequency used forexchanging a discovery signal. The frequency used for transmitting adiscovery signal and the frequency used for receiving a discovery signalmay differ from each other.

For example in other embodiments, a program may be provided for causinga computer to execute each process performed by the UE 100 and the eNB200. Furthermore, the program may be recorded on a computer-readablemedium. By using the computer-readable medium, it is possible to installthe program in a computer. Here, the computer-readable medium recordingthe program thereon may include a non-transitory recording medium. Thenon-transitory recording medium is not particularly limited. Forexample, the non-transitory recording medium may include a recordingmedium such as a CD-ROM or a DVD-ROM.

Alternatively, a chip may be provided which is configured by: a memoryin which a program for performing each process performed by the UE 100and the eNB 200 is stored; and a processor for executing the programstored in the memory.

In the embodiments, the LTE system is described as an example of themobile communication system. However, the embodiments are not limitedthereto. The mobile communication system may be a system other than theLTE system.

[Additional Statements]

Below, supplementary notes of the embodiments will be additionallydescribed.

(1. Introduction)

The below additional statements provide analysis about inter-frequencydiscovery.

(2. Deployment Scenario for Inter-Frequency Discovery)

Possible deployment scenario for inter-frequency discovery isillustrated in FIG. 6 to FIG. 8, where UE 1 is camped on/connected toCell 1 operating on f1, and UE 2 is camped on/connected to Cell 2operating on f2. The eNBs operating Cell 1 or Cell 2 may/may not operateSCell on f2 or f1.

The scenario can be categorized into three cases depending on whetherintra-eNB/homogeneous network (HomoNet), inter-eNB/heterogeneous network(HetNet) and inter-eNB/HomoNet is deployed. These cases are depicted inFIG. 6 to FIG. 8, respectively. Actual deployments may consist of acombination of the categorized cases.

For the intra-eNB/HomoNet case depicted in FIG. 6, it may includeintra-eNB carrier aggregation (CA) deployment, i.e. the UEs areCA-capable but the PCells are configured with different frequencies,i.e. UE 1's PCell is Cell1 on f1 and UE 2's PCell is Cell 2 on f2. Thesimplest case is, for example, the SCell for UE 1 is configured withCell 2, and eNB 1 allows D2D operations on Cell 2. In this case, UE 1and UE 2 will be able to perform D2D over f2 without too much problem.

For the inter-eNB/HetNet case, in FIG. 7, D2D operation is a slightlymore complex since the RRC connections of the UEs are established overdifferent eNBs. If UE 2 is assumed as dual connectivity (DC)-capable UEand configured with f1 for RRC connection and with f2 for secondaryresource, then UE 2 will be able to perform D2D with UE 1 over f1 sinceUE1 is only served on f1.

The common point in the two cases above is that both UEs havepossibilities to be provided a common coverage cell on a frequency.Therefore, one way to support inter-frequency discovery is to ensurethat all D2D UEs are served under the same overlaid cell under the samefrequency.

Observation 1: If D2D UEs are served under the same overlaid cell,inter-frequency discovery can be easily supported.

For the inter-eNB/HomoNet case, as depicted in FIG. 8, D2D operationwill involve the most complicated network planning, since there isn't anoverlaid layer for both D2D UEs and standardized coordination over X2 isdisallowed in Rel-12.

Note that for synchronous deployment, Cell 1 and Cell 2 are assumed tobe time synchronized. The issue with D2D synchronization under theasynchronous deployment is discussed separately.

Proposal 1: it should assumed that at least for Rel-12 that the UEsattempting inter-frequency discovery are provided with at least anoverlaid cell operated on one frequency.

Up to now, RAN2 has not discussed the case whereby only a subset of thedeployed operating frequencies may support D2D discovery. Although thismay be considered as eNB implementation or operator's preference, thisdoes have an impact on inter-frequency, inter-cell discovery. Forexample, assuming the case with three operating frequencies, the supportfor D2D may be categorized into three plans as shown in Table 1.

TABLE 1 f1 f2 f3 Plan (e.g. 800 MHz) (e.g. 2 GHz) (e.g. 3.5 GHz) Note 1D2D/WAN D2D/WAN D2D/WAN Assume D2D demands growth 2 D2D/WAN D2D/WAN WANonly 3 D2D/WAN WAN only WAN only Assume initial deployments

Plan 1 assumes all of the operating frequencies allow D2D operationwhich may or may not include D2D discovery or D2D communication. Plan 2assumes two of the three operating frequencies allow D2D operation.These two plans above will require some mechanism for inter-frequencydiscovery.

With plan 3, only a single frequency supports D2D operation and it maybe considered as just an intra-frequency scenario. However, in the plan3 scenario, it is assumed that not all D2D UEs are initially served bythe cell(s) on the same frequency; therefore, some mechanism(s) may beneeded to ensure that all D2D UEs can operate D2D on the one frequencyavailable for D2D.

As the main purpose for inter-frequency discovery is to enable the UE tobe served under a D2D allowable frequency, the assumption is that theUE's current serving frequency may not be one of the D2D allowablefrequencies. In order to assist the UE in tuning to the proper frequencyfor monitoring/transmitting D2D discovery, it should be possible for thenon-D2D serving cell to provide the list of neighboring frequencies thatdo support D2D. This is similar to the scenario for MBMS whereby bothMBMS and non-MBMS cells indicate in SIB15 the MBMS SAIs of the currentfrequency and of each neighbor frequency.

Proposal 2: D2D allowable frequencies should be provided by the SIB ofthe serving cell, regardless of whether the serving cell allows D2Doperation.

(3. Transmission and Reception Scheme Alternatives)

Taking into account the possible deployment scenario, inter-frequencydiscovery may be accomplished with one or more of the three alternativesbelow.

ALT 1: UE 1 transmits a discovery signal on f1, then UE 2 receives thesignal on f1, i.e. inter-frequency discovery cell reception mechanism.In this alternative, UE 2 is assumed to have at least a receiver forboth frequencies. For more efficient operation, it may be assumed thatthe UE has dual receivers and/or Carrier Aggregation capabilities.

ALT 2: UE 1 transmits a discovery signal on f2, e.g. inter-frequencydiscovery transmission mechanism, then UE 2 receives the signal on f2.In this alternative, UE 1 may be assumed to have at least a transmitterfor both frequencies. For more efficient operation, it may be assumedthat the UE has dual transmitters and/or Carrier Aggregationcapabilities.

ALT 3: UE 1 transmits a discovery signal on f1, then UE 2 receives thesignal on f1 after it is handed over to f1. In this alternative, the eNBoperating Cell 2 is assumed to have another cell that can be operated onf1.

ALT 1 is a straightforward scheme since Cell 1 allocates only resourceswithin its own operating frequency to UE 1 for the transmission ofdiscovery signals, while UE 2 will need to receive the discovery signalon a frequency different from the serving frequency.

ALT 2 has the potential for more flexibility in network planning,assuming the multi-carrier D2D operation is supported, e.g. plan 1 inTable 1. While the benefit is expected for D2D communication, especiallyfor the unicast case, for Rel-12 this alternative only results inunnecessary complexity, since the discovery and the 1:many D2Dcommunication are assumed for the broadcast transmission/reception, i.e.the transmitted signal should be received by all D2D-enabled UEs withinrange.

ALT 3 is actually a mechanism to try and reuse the intra-frequency D2Ddiscovery as much as possible under the multi-frequency deploymentscenario. Due to the reuse of existing intra-frequency D2D discoverymechanism, ALT 3 may result with the least impact to the UE, since D2Ddiscovery is only performed on the common frequency which allows D2Doperation, i.e. plan 3 in Table 1.

Proposal 3: RAN2 should preclude the scheme for the discovery signal tobe transmitted on a frequency different from the serving frequency forRel-12.

(4. Details of the Scheme Alternatives)

(4.1. ALT 1 Details)

(4.1.1. Reception of Inter-Frequency Discovery Signals)

For the intra-frequency discovery, both intra-cell and inter-cellscenarios have been discussed in RAN2 and several agreements werereached during the study item phase. Some discussions have also startedunder the work item phase. As for inter-cell discovery, RAN2 reached thefollowing agreement on D2D reception discovery resources.

The eNB may provide D2D reception discovery resources in SIB. These maycover resources used for D2D transmission in this cell as well asresources used in neighbor cells.

In addition, RAN1 agreed with the following aspects.

Confirm that radio resources pool(s) may be provided by eNB for D2D UEsin SIB for discovery reception for Type-2B (if supported). It needsfurther study whether the common reception pool(s) or differentreception pools for type 1 and Type-2B discovery. A UE is not requiredto decode neighboring cell SIB.

Mechanisms for Type-2B discovery. A resource hopping mechanism followingthe resource allocation by eNB can be applied. Details of resourcehopping mechanism should be studied.

The agreements above intended that the SIB transmitted from the servingcell provides the information to UEs for reception of inter-celldiscovery signals. Therefore, the UE can receive the signals withoutdecoding any SIB transmitted from the neighbour cells.

Observation 2: With the reception pool(s) provided by the SIB from theUE's serving cell, the UE may receive the discovery signal withoutdecoding the neighbour cells's SIB.

The term “neighbour cell” typically includes a neighbour cell on thesame frequency as well as on a different frequency. However, thediscussions towards the above agreement for resource allocation in RAN2have not clearly considered the inter-frequency discovery scenario.Therefore, the details should be discussed and defined.

Assuming the agreements includes the neighbour cells on differentfrequencies, the SIB transmitted by the serving cell should include D2Ddiscovery resources on the serving frequency and neighbour frequencies.With the available discovery resources provided over SIB, D2D UEs willbe able to receive D2D discovery signals from UEs on a neighbouringinter-frequency cell. We assume it may be possible to add some extensionparameters to the information element (IE) to be defined for inter-celldiscovery and to re-use existing inter-frequency measurement mechanism,to support inter-frequency discovery using ALT 1.

Proposal 4: As the baseline, RAN2 should consider re-using existinginter-frequency measurement mechanism for inter-frequency discovery, ina case where ALT 1 is supported in Rel-12.

(4.1.2. Possible Issues)

(4.1.2.1. Measurement Gap Length and Triggering)

According to the table 10.1-2 in the TR, RAN1 assumed 16-64 subframesper discovery period for performance evaluation in the study phase. Forinter-frequency measurements, the existing measurement gap length isfixed at 6 subframes. Without any further enhancement, the detectionprobability for discovery, i.e. the opportunity to discover D2D UEswould be degraded to at least ⅓.

With respect to the timing of the configuration of measurement gaps, therelationship between RRM related measurements (i.e., RSRP/RSRQ) of theeNBs and the received discovery signal power from the other D2D UEs maynot be directly correlated. In other words, a D2D UE may receivediscovery signals from D2D UEs in a neighbour cell even though the D2DUE is nowhere close to the neighbour cell. Considering theinter-frequency discovery with ALT 1, there is no reason to deny the D2DUE from the continuous monitoring of D2D discovery, although thedecision to assign gaps of discovery should be left for eNBimplementation. If D2D UEs have no interest in receiving/transmittingD2D discovery, the D2D UEs should inform the serving eNB of the statusof its D2D operation; i.e., whether D2D operation is disabled by theuser.

Observation 3: In a case when the existing mechanism for inter-frequencymeasurement is re-used, the detection probability will be degraded.

Observation 4: While RRM related measurements of the eNBs and thereceived discovery signal power from the other D2D UEs may not bedirectly correlated, it should be the eNB responsibility to assign theUE with gaps to monitor inter-frequency discovery signal.

Based on Observation 3, it could be considered whether additionalenhancements are needed for inter-frequency discovery e.g., to define anew “discovery monitoring gap” with larger number of subframes than theexisting measurement gap length.

However, it will result in less opportunity for WAN UL/DL transmissionson the camped/served frequency, while the detection probability ofdiscovery will improve.

Observation 5: If the gap is enhanced for inter-frequency discovery, aUE will lose the opportunity for cellular operations, due larger numberof inter-frequency monitoring and/or longer discovery subframes.

Proposal 5: RAN2 should take into account the trade-off between thedetection probability of discovery and the opportunity of WAN UL/DLtransmissions, in case ALT 1 is supported.

To reduce the impact of the trade-off above, the simultaneous operationof inter-frequency measurement and inter-frequency discovery should beconsidered, for the case where the inter-frequency neighbouring cellsfor discovery are the same cells or a part of the cells for handover,i.e. for normal cellular operation. Since the inter-frequencymeasurement needs a DL receiver and does not require a UL transceiver,i.e. D2D receiver is free during the gap; therefore, the UE can receivediscovery signal during the same gap.

Observation 6: Inter-frequency monitoring of discovery signals may beperformed during existing measurement gap and performed together withinter-frequency measurement simultaneously.

Proposal 6: As the baseline, if ALT 1 is chosen, RAN2 should assumeinter-frequency monitoring of discovery signals is performed togetherwith existing inter-frequency measurement using the same measurementgap.

(4.1.2.2. UE Capability)

Thus far we have assumed that UEs with a single D2D transceiver wouldonly have the capability to operate on one frequency at any given time,regardless of number of supported frequencies for WAN communication asindicated in the plans in Table 1.

Note that the frequencies for WAN communication mean carriers includingintra-band and inter-band.

Observation 7: The D2D transceiver should not be assumed to work on allfrequencies supporting WAN communication.

To facilitate more flexible use of D2D, CA-capable UEs may be consideredfor inter-frequency discovery. In this case, the CA-capable UEs cantransmit/monitor discovery signals on PCell and SCells simultaneously.Therefore, the “discovery monitoring gap” will not be needed for such aUE as long as the D2D discovery frequency belongs to one of frequenciesof the UE's serving cells (i.e., PCell or SCell).

Observation 8: For carrier aggregation capable UEs, inter-frequencydiscovery can be performed without assigning measurement gaps, as longas the D2D discovery frequency belongs to one of frequencies of the UE'sserving cells

(4.2. ALT 3 Details)

(4.2.1. Handover Procedure for D2D Discovery)

Alternative 3 is mainly applicable to plan 2 or 3 in Table 1, i.e., notall cellular frequencies support D2D. This alternative assumes handoveris completed before discovery is initiated. In particular, if aD2D-enabled UE is served on a frequency that does not support D2Doperation, the eNB should handover the UE to a target cell operating ona D2D allowable frequency.

The potential benefits are as follows.

If the SIB provided on the frequency not allowing D2D operation does notinclude any D2D-related information, e.g. discovery reception resources,then the UE camped/served on the frequency has no way totransmit/receive discovery signals on the different frequency. This is alikely scenario for stand-alone small cell deployment, e.g. the case inFIG. 7, since currently there is no agreement whether the non D2Doperating cell shall provide the D2D-related information in its SIB.

Even if D2D discovery resources are provided in a SIB for a cell notsupporting D2D, the cell will not be able to allocate Type-2B discoveryresource to the UE since the cell does not support any D2D operations.

Observation 9: If the frequency of the UE's serving cell does not allowD2D operation, the UE may lose the opportunity to initiate D2Ddiscovery.

However, it also has some drawbacks.

If multiple frequencies support D2D operation, ALT 3 should be combinedwith ALT 1 or ALT 2.

If all of D2D-capable UEs are served on limited frequencies whichsupport D2D operation, it may result in congestion on the frequencies.

(4.2.2. Possible Issues)

(4.2.2.1. Trigger to Handover for Discovery)

In a case all D2D-capable UEs are served on the same cell, e.g. the plan3 in Table 1, the cell may suffer from congestion, e.g. less opportunityfor load balancing in WAN communication brought by multiple frequencydeployment. To avoid this issue, only D2D-enabled UEs, which are a partof D2D-capable UEs, should be considered to be handed over to the D2Dallowed cell, and when the UE disables D2D then it should be up to eNBimplementation to decide whether the D2D-disabled UE remains in the cellor is handed over to the other cell, based on e.g. current load in thecell.

Observation 10: When the UE enables D2D functionality, the eNB mayhandover the UE to a cell which allows discovery.

According to the TR of SA1, the discovery feature can beenabled/disabled by a user as follows.

5.1.1.5 Potential Requirements. General. [PR.1] Based on operator policyand user choice, the proximity of two ProSe-enabled UEs shall bedeterminable; for example, using direct radio signals or via theoperator network. [ . . . ] [PR.3] Operator policy and user choice canset the ProSe feature of: [ . . . ] a ProSe-enabled UE to disable theability to be discoverable by other UEs and to disable the ability todiscover other UEs; [ . . . ] [PR.97] Operator policy disabling orlimiting individual ProSe features shall override any user choice. [ . .. ]

Thus, whether the UE enables discovery functionality will depend on userpreference. We assume D2D-capable UE will support a user interface toallow the user to enable/disable the discovery functionality.

Observation 11: Whether the UE enables discovery functionality willdepend on user preference.

In this case, eNB should have a capability to know whether the UEenables its discovery function or not. In our view, this use case isvery similar with the concept of the existing MBMS interestingindication indicating an interest for MBMS, therefore, we propose tointroduce an MBMS-like solution to trigger inter-frequency handover,i.e. using “D2D interest indication”.

Proposal 7: The UE should have the capability to inform the eNB when theUE enables/disables its discovery functionality.

(4.2.2.2. Discovery in IDLE Mode)

Although the method proposed in section 4.2.2.1 is useful for the UEs inRRC connected, the same is not true for UEs in idle. IDLE UEs would berequired to transition to CONNECTED, send D2D interest indication, andget handed over to a D2D capable cell before D2D discovery is possible.

In order to achieve similar efficiency for UEs to receive/transmit D2Ddiscovery signals, IDLE UEs should be allowed to prioritize D2Dfrequency as part of the reselection procedure. Currently, the frequencypriority for reselection is determined by the eNB; however, if IDLE UEsare interested in D2D, it should be possible for IDLE UEs to prioritizeD2D frequency. This idea is similar to the current MBMS behavior wherebyIDLE UEs interested in MBMS are allowed to prioritize MBMS frequency forreselection.

Proposal 8: IDLE UEs with D2D enabled should be allowed to prioritize aD2D frequency for reselection.

If Proposals 2 and Proposal 8 are agreeable, an IDLE UE may prioritizeD2D frequency based on the D2D information provided in the SIB of itsserving cell.

(4.3. Comparison of the Alternatives)

As a summary, we provided a comparison of the alternatives as shown inTable 2.

TABLE 2 Alternative 1 Alternative 2 Alternative 3 Method D2D receptionon D2D transmission on Handover/reselection different frequencydifferent frequency before discovery from serving frequency from servingfrequency Complexity/ Medium High Low Standard impact Based on gap (notanalyzed) Based on MBMS-like assignment mechanism similar to RRMFlexibility Medium potentially high Low Drawback Trade-off between (notanalyzed) D2D operating WAN DL/UL frequency may be limited opportunityand and increased network discovery probability. congestion is possible.Proposal Should be supported Should be excluded May be considered forRel-12 from Rel-12 for Rel-12

Proposal 9: RAN2 should consider if one or more of the abovealternative(s) should be adopted in Rel-12 to support inter-frequencydiscovery.

INDUSTRIAL APPLICABILITY

The present disclosure is useful for communication fields.

1. A user terminal located inside the coverage area of a first cell, the first cell operating at a first frequency in a mobile communication system that supports a D2D (Device to Device) proximity service, the user terminal comprising: a processor configured to transmit a D2D interest indication to a base station operating in the first cell, the D2D interest indication including information indicating a second frequency in which the user terminal has an interest in the D2D proximity service, wherein the transmission of the D2D interest indication enables the base station to determine one of the first cell and a second cell where the user terminal will operate to obtain the D2D proximity service, wherein the D2D proximity service includes a D2D discovery signal step for discovering other user terminals located in one of the first cell operating at the first frequency and the second cell operating at the second frequency; and transmit a D2D discovery signal of the D2D discovery signal step to another user terminal in one of the first cell and the second cell where D2D resources are available.
 2. The user terminal according to claim 1, wherein the processor is configured to transmit the D2D interest indication, upon a condition that the D2D resources are available at the second cell operating on the second frequency.
 3. The user terminal according to claim 2, wherein the processor is configured to receive a SIB (System Information Block) notified from the base station forming the first cell operating at the first frequency, the SIB including information indicating that the D2D resources are available on the second cell operating at the second frequency.
 4. The user terminal according to claim 1, wherein the processor is configured to receive a handover command from the base station to handover the user terminal to the second cell operating on the second frequency, wherein D2D resources are available only in the second cell operating in the second frequency.
 5. The user terminal according to claim 1, wherein obtaining the D2D proximity service in one of the first cell and the second cell includes determining the availability of the D2D resources.
 6. The user terminal according to claim 1, wherein as a result of the D2D interest indication transmitted to the base station, the user terminal does not receive a handover command to the second cell operating on the second frequency when the second cell does not provide the D2D resources.
 7. The user terminal according to claim 1, wherein the D2D resources are provided only in the second cell operating in the second frequency.
 8. The user terminal according to claim 1, wherein the user terminal determines which other user terminals with which the user terminal wants to perform D2D proximity service without assistance from the base station.
 9. An apparatus for controlling a user terminal, the user terminal located inside the coverage area of a first cell, the first cell operating at a first frequency in a mobile communication system that supports a D2D (Device to Device) proximity service, the apparatus comprising: a processor configured to transmit a D2D interest indication to a base station operating in the first cell, the D2D interest indication including information indicating a second frequency in which the user terminal has an interest in the D2D proximity service, wherein the transmission of the D2D interest indication enables the base station to determine one of the first cell and a second cell where the user terminal will operate to obtain the D2D proximity service, wherein the D2D proximity service includes a D2D discovery signal step for discovering other user terminals located in one of the first cell operating at the first frequency and the second cell operating at the second frequency; and to transmit a D2D discovery signal of the D2D discovery signal step to another user terminal in one of the first cell and the second cell where D2D resources are available.
 10. The apparatus according to claim 9, wherein the processor transmits the D2D interest indication, upon condition that the D2D resources are available at the second cell operating on the second frequency.
 11. The apparatus according to claim 10, wherein the processor receives information notified from the base station forming the first cell operating at the first frequency, the information indicating that the D2D resources are available on the second cell operating at the second frequency.
 12. The apparatus according to claim 9, wherein the processor receives a handover command from the base station to handover the user terminal to the second cell operating on the second frequency, wherein the D2D resources are available only in the second cell operating in the second frequency. 